Molecular sieves are usually obtained from industrial production in the form of crystalline powders with a fine particle size. These molecular sieves must, therefore, be shaped into larger agglomerates for most applications. For their use as drying media, problems in shaping often arise because granules of high mechanical strength must be produced without reducing the adsorption capability of the molecular sieve.
In most of the known processes for the production of shaped bodies from molecular sieves water glass and/or clay are used as binders. Even though the bodies obtained may possess adequate mechanical strength, their water adsorption capability is reduced. Thus, West German Published Application No. 1 192 164 describes a process for the manufacture of molecular sieves in a spherical shape, wherein finely crystalline molecular sieves are mixed with water glass to form a paste and dripped into an aqueous solution of sodium, alkaline earth metal, nickel, cobalt or aluminum salts as the solidification liquid. The spheres obtained by the process have good water absorption capabilities, but their mechanical strength is entirely inadequate.
According to the process described in West German Published Application No. 1 165 562, the granulation of molecular sieves is attained by stirring them with a silicic acid sol into a flowable suspension having a pH value of 8.0 to 10.0, preferably 8.2 to 9.0, mixing the suspension with small amounts of a suspension of magnesium oxide and dripping the mixture into a liquid immiscible with water; the granules are then dried and hardened by means of a heat treatment. The adsorption capacity of the granules obtained in this manner corresponds to the adsorption capacity of the zeolite contained therein. The difficulty inherent in the process resides in the fact that the zeolite-silica sol must be adjusted to a narrow pH range. This is because in the case of excessively low alkalinity, the zeolite solution converts at a relatively rapid rate into a soft gel; at only slightly higher pH values, on the other hand, magnesium oxide loses its effectiveness as a gelling agent completely. Molecular sieves must therefore be freed of appreciable amounts of alkaline contamination by means of a thorough washing. In addition, the spheres of the gel have no wet strength prior to hardening. Special measures must therefore be taken to prevent their agglomeration and their adherence to the walls of vessels, a condition which eventually may lead to incrustations.